1. Field of the Invention
This invention relates generally to magnetic recording hard disk drives (HDDs), and more particularly to a HDD that has adjustable data track pitch.
2. Description of the Related Art
HDDs typically have disks with data tracks with a fixed track spacing or pitch that is set during manufacturing and cannot be changed during the life of the HDD. The positioning of the read/write heads to the data tracks is accomplished by servo tracks that have angularly-spaced servo sectors that contain head positioning information. The servo sectors extend radially across the data tracks. The read head detects the positioning information as the disk rotates and passes the position information to a servo control system to maintain the head on the desired data track. The servo tracks also have a fixed track pitch that is set during manufacturing and that may be different from the fixed track pitch of the data tracks.
A head following the center of a servo track moves along a perfectly circular path around the disk. In reality, however, as a head attempts to follow a servo track it will not follow a perfect circular path. Instead, the head will deviate from the desired circular path due to a variety of factors, including disk irregularity, disk eccentricity, and/or misalignments of the spindle axis assembly. This deviation from the desired path is referred to as “runout.” Repeatable runout (RRO) refers to deviation that is non-random and occurs with some predictability. During disk drive manufacturing, RRO can be measured and corresponding RRO offset values for each servo sector in each servo track can be determined. Such RRO correction data can be stored in the disk drive. In HDDs with data tracks having a fixed track pitch, this RRO information is stored in the data tracks.
In certain HDDs it may be desirable to change the data track pitch during the life of the HDD. For example, the HDD may be operating in a high-vibration environment, in which case it may be desirable to increase the data track pitch. A HDD with adjustable data track pitch to compensate for vibration is described in U.S. Pat. No. 8,724,248 B2, assigned to the same assignee as this application. However, the prior art method of storing RRO fields in the data tracks will not work in a HDD with adjustable data track pitch because the old data tracks will be overwritten when the new data tracks with a different track pitch are written.
What is needed is a magnetic recording disk drive with adjustable data track pitch that can compensate for RRO of the servo tracks.